1. Field of the Invention
This invention relates to a thermal fixing apparatus for fixing an image formed on a recording medium by heating it, and an image forming apparatus having said thermal fixing apparatus. The invention further relates to a fixing process for fixing an image on a recording medium.
In an image forming apparatus such as an ink jet recording apparatus, etc., in order to cause discharged ink to be fixed on a recording form such as a paper, film or the like (hereinafter, called a "recording medium") by drying the ink on the recording medium, many drying methods have used including controlling the conveying speed of the recording medium, emitting the recording medium after holding it at the recording position during a predetermined period of time after the completion of recording, and advancing the recording medium on a recording medium conveying path heated by a heater or the like to heat the recording medium. Particularly, the heating method for drying is noticeably recognized as what is most effective in fixation.
2. Related Background Art
FIG. 1 schematically shows a fixing unit for carrying out the fixing for a recorded image by the heating method. A recording medium 100 is conveyed or fed onto a thermal conveying path 103 heated by a heater (not shown), by two sets of conveying roller pairs 101 and two sets of form emitting or exhausting roller pairs 102, each set of roller pairs 101 and 102 being positioned at one of two image blank portions at the right and left edges of the recording medium 100. Usually, the conveying speed of the respective form emitting roller pairs 102 is set to be higher than that of the respective conveying roller pairs to give tensile force to the recording medium 100 between the roller pairs 101 and 102 in the form advancing direction (shown by arrow A), so that the recording medium can be closely contacted with the thermal conveying path 103 to provide improved fixing capability due to good thermal transferability.
However, in case where a resin or plastic film like sheet such as an OHP form (transparency) which typically comprises resin sheet material having an ink accepting layer, is used as the recording medium 100, when it contacts with the thermal conveying path 103 in the course of its advancing on the thermal conveying path 103, the film sheet should be often overheated owing to inappropriate temperature setting for the thermal advancing path, which leads the film sheet to its deformation, so that the film sheet would produce a "waving phenomenon" as shown in FIG. 2. Even though the thermal conveying path 103 is set to be such temperature that occurrence of said waving phenomenon due to overheating would be avoided, the recording medium 100 receives force acting toward its small expansion as it passes through the thermal conveying path 103. Under this condition, after the recording medium 100 has passed through the thermal conveying path 103, the recording medium receives force acting toward its contraction since the heat thereof is radiated. These two forces which are contrary to each other would be produce the waving phenomenon shown in FIG. 2. Since this waving phenomenon cannot be easily suppressed by merely applying, to the recording medium, the tensile force in the conveying direction for the recording medium, the sheet film itself would be deformed upon heating, and the raised portions of the film from the heat conveying path would not be maintained in a condition in which they are sufficiently heated, and accordingly, parts of the image on the film would not be dried sufficiently and left unfixed. Therefore, there would be reproduced a disturbed image.